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x86/boot/compressed/64: Switch to __KERNEL_CS after GDT is loaded

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When the pre-decompression code loads its first GDT in startup_64(), it
is still running on the CS value of the previous GDT. In the case of
SEV-ES, this is the EFI GDT but it can be anything depending on what has
loaded the kernel (boot loader, container runtime, etc.)

To make exception handling work (especially IRET) the CPU needs to
switch to a CS value in the current GDT, so jump to __KERNEL_CS after
the first GDT is loaded. This is prudent also as a general sanitization
of CS to a known good value.

 [ bp: Massage commit message. ]

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200428151725.31091-13-joro@8bytes.org
This commit is contained in:
Joerg Roedel 2020-04-28 17:16:22 +02:00 committed by Borislav Petkov
parent 40ba9309c7
commit 34bb49229f
1 changed files with 8 additions and 0 deletions
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8
arch/x86/boot/compressed/head_64.S
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@ -393,6 +393,14 @@ SYM_CODE_START(startup_64)
addq %rax, 2(%rax)
lgdt (%rax)
/* Reload CS so IRET returns to a CS actually in the GDT */
pushq $__KERNEL_CS
leaq .Lon_kernel_cs(%rip), %rax
pushq %rax
lretq
.Lon_kernel_cs:
/*
* paging_prepare() sets up the trampoline and checks if we need to
* enable 5-level paging.